Add test demoing the use of function call for Org keyword value

Thanks @xeijin for this idea!

Ref: https://github.com/kaushalmodi/ox-hugo/issues/272

test/issue-272-chkbox-items-to-front-matter.el

@@ -0,0 +1,47 @@
+(defun xeijin/conv-chkbox-items-to-front-matter (hl)
+  "Find the headline exactly matching HL.
+
+Then find all plain list items under HL and return as a
+list \\='((checked . (VALa VALb ..)) (not-checked . (VALx VALy
+..))).
+
+- The values in \"checked\" cons are the Org list items with
+  checkbox in \"on\" state.
+
+- The value in \"not-checked\" cons are the Org list items with
+  any other checkbox state, or no checkbox."
+  ;; (message "dbg x: pt: %d" (point))
+  (let (hl-as-element
+        checked not-checked
+        ret)
+    (save-restriction
+      (ignore-errors
+        (org-narrow-to-subtree)) ;This will give error when there's no
+                                        ;heading above the point, which will
+                                        ;be the case for per-file post flow.
+      (save-excursion
+        (goto-char (point-min))
+        ;; (message "dbg y: pt: %d" (point))
+        (let (case-fold-search) ;Extracted from `org-find-exact-headline-in-buffer'
+          (re-search-forward
+	   (format org-complex-heading-regexp-format (regexp-quote hl)) nil :noerror))
+        ;; (message "dbg z: pt: %d" (point))
+        (save-restriction
+          (org-narrow-to-subtree) ;Narrow to the `hl' headline
+	  (setq hl-as-element (org-element-parse-buffer)))
+        ;; (message "dbg: %S" hl-as-element)
+        (org-element-map hl-as-element 'item ;Map over headline's items
+	  (lambda (item)
+	    (let* ((checkbox-state (org-element-property :checkbox item)) ;Get checkbox value of item
+		   (item-text (string-trim (substring-no-properties
+                                            (org-element-interpret-data
+                                             (org-element-contents item))))))
+              (cond
+               ((eq checkbox-state 'on)
+                (push item-text checked))
+               (t ;checkbox state in `off' or `trans' state, or if no checkbox present
+                (push item-text not-checked))))))
+        (setq ret `((checked . ,(nreverse checked))
+                    (not-checked . ,(nreverse not-checked))))))
+    ;; (message "dbg: ret: %S" ret)
+    ret))

test/setup-ox-hugo.el

@@ -269,5 +269,10 @@ Fake current time: 2100/12/21 00:00:00 (arbitrary)."
   (advice-add 'current-time :override #'ox-hugo-test/current-time-override)
   ;; (advice-remove 'current-time #'ox-hugo-test/current-time-override)
 
+  ;; issue # 272
+  (load (expand-file-name "test/issue-272-chkbox-items-to-front-matter.el"
+                          ox-hugo-site-git-root)
+        nil :nomessage :nosuffix)
+
   (with-eval-after-load 'ox
     (add-to-list 'org-export-exclude-tags "dont_export_during_make_test")))

test/site/content-org/all-posts.org

@@ -2010,6 +2010,35 @@ TOML front-matter.
 But the big integer "10040216507682529280" which would need more than
 64-bits to be stored as a signed integer is left as a string in the
 TOML front-matter.
+** Custom front matter value via Elisp                                :elisp:
+:PROPERTIES:
+:EXPORT_HUGO_CUSTOM_FRONT_MATTER: :attendees (xeijin/conv-chkbox-items-to-front-matter "Attendees")
+:EXPORT_AUTHOR: xeijin
+:END:
+*** Meeting 1
+:PROPERTIES:
+:EXPORT_FILE_NAME: custom-fm-convert-chkbox-list-to-fm-1
+:END:
+{{{oxhugoissue(272)}}}
+**** Attendees                                                     :noexport:
+- [X] Attendee 1
+- [-] Attendee 2
+- [ ] Attendee 3
+- Attendee 4
+**** Notes
+My notes
+*** Meeting 2
+:PROPERTIES:
+:EXPORT_FILE_NAME: custom-fm-convert-chkbox-list-to-fm-2
+:END:
+{{{oxhugoissue(272)}}}
+**** Attendees                                                     :noexport:
+- [X] Attendee x
+- [-] Attendee y
+- [X] Attendee z
+- Attendee w
+**** Notes
+My notes
 * Resources                                                       :resources:
 ** TOML                                                                :toml:
 *** Post with resources in front-matter (TOML)

test/site/content/posts/custom-fm-convert-chkbox-list-to-fm-1.md

@@ -0,0 +1,16 @@
++++
+title = "Meeting 1"
+author = ["xeijin"]
+tags = ["custom-fm", "elisp"]
+draft = false
+[attendees]
+  checked = ["Attendee 1"]
+  not-checked = ["Attendee 2", "Attendee 3", "Attendee 4"]
++++
+
+`ox-hugo` Issue #[272](https://github.com/kaushalmodi/ox-hugo/issues/272)
+
+
+## Notes {#notes}
+
+My notes

test/site/content/posts/custom-fm-convert-chkbox-list-to-fm-2.md

@@ -0,0 +1,16 @@
++++
+title = "Meeting 2"
+author = ["xeijin"]
+tags = ["custom-fm", "elisp"]
+draft = false
+[attendees]
+  checked = ["Attendee x", "Attendee z"]
+  not-checked = ["Attendee y", "Attendee w"]
++++
+
+`ox-hugo` Issue #[272](https://github.com/kaushalmodi/ox-hugo/issues/272)
+
+
+## Notes {#notes}
+
+My notes